The influence of cardiolipin on phosphatidylglycerol/phosphatidylethanolamine monolayers—Studies on ternary films imitating bacterial membranes

In this work the properties of ternary systems composed of phosphatidylethanolamines (PEs), phosphatidylglycerols (PGs) and cardiolipin (CL) were studied with the Langmuir monolayer technique and Brewster Angle Microscopy. In all the investigated mixed films the PE:PG = 3:1, which reflects the proportion of these lipids classes in various Gram Negative bacteria. The content of cardiolipin was varied from 5 to 20%, which is in the range of CL concentration in bacterial membranes. The experiments were performed for POPE/POPG/CL and POPE/DPPG/CL films spread on water subphase and on NaCl solution. It was found that the components of the studied films mix nonideally in the investigated range of the composition, however, the addition of cardiolipin weakens the interactions between the molecules. Thermodynamically unfavorable influence of CL results from the structure of this lipid, which due to sterical reasons disturbs the organization of POPE/PG film. It was also found that the presence of sodium ions in the subphase causes the increase of the area per molecule values and in general weakens the interactions between molecules in ternary films. However, the foregoing effect of NaCl both on binary PE/PG films as well as ternary PE/PG/CL monolayers was stronger on DPPG-containing films. The analysis of the collected data evidenced that the influence of cardiolipin as well as inorganic electrolyte on the properties of the studied films is directly connected with molecular structure of monolayers components.